The aim of the current study was to further identify the factor in antiphospholipid (APL) positive serum, which causes the local detrimental effect on fetal survival in the previously reported experimental rat model. Serum samples, obtained from women with APL antibody syndrome, were heated to 100% for 10 minutes, centrifuged, and the supernatant was separated on a 30 kD cutoff membrane. The flow through fraction was used for injection into unilateral horn of the rats on day L₅ ­ of rat pregnancy. The contralateral uterine horn was injected with a control sample. The boiled APL positive serum fraction, lower than 30 kD, caused a significant increase in fetal rat resorption rate as compared with similarly prepared normal serum and saline. Moreover, the ten fold diluted fraction, obtained from boiled APL positive serum, still demonstrated a significant increase in fetal resorption rate (93.1%) as compared with similarly diluted fraction, obtained from boiled normal serum and saline. This study demonstrates a probable existence of another factor in APL positive serum, besides the complex of antiphospholipid antibodies with a cofactor, which causes a local detrimental effect on fetal survival in the experimental rat model.